ASoC: sgtl5000: Place optional properties in the correct section

'micbias-resistor-k-ohms' and 'micbias-voltage-m-volts' are optional
properties, so move them below the 'Optional properties' line.

While at it, fix a typo in 'mentioned'.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>